1. Field of the Invention
The invention relates to a magnet arrangement for generating an NMR-compatible homogeneous permanent magnetic field.
2. Description of the Prior Art
NMR-imaging methods and also corresponding devices are used, inter alia, for the nondestructive material and structural study of objects which may comprise crystalline, glassy materials or soft materials, such as elastomers, or liquids, or biological materials.
A known NMR method and also a device for this purpose is described in DE 199 39 626 A1, in which a transportable NMR device is described. The known device provides two permanent magnets spaced apart from one another, which are used for generating a stationary magnetic polarization field. An HF coil arrangement and two gradient coils are provided between the permanent magnets, and the permanent magnets are used for generating a magnetic gradient field superimposed on the stationary magnetic polarization field, which is pulse operated at time intervals for the locally-resolved NMR measurement.
With the aid of the HF coil arrangement, which is also used as a receiving coil, in addition to generating the HF alternating field, high-frequency signals, which are caused by nuclear resonance processes occurring in the interior of the sample, are received and graphically displayed using an analysis unit. The nuclear resonance processes occurring in the interior of the sample are predominantly to be attributed to nuclear spin interactions of the hydrogen atoms with the stationary magnetic polarization field, which originate from a targeted energetic HF excitation by the HF alternating field, and may be related in the analysis in the broadest sense to the hydrogen concentration present inside the sample volume being studied.
The previously described NMR device does allow a planar study of an object by sliding over the object surface in the implementation of a transportable device, but the analyzable object volume is restricted to only a few millimeters by the very slight penetration depth of the magnetic fields which can be generated by the NMR device. It is certainly possible to improve the penetration depth into the object to be studied by corresponding size dimensioning of the permanent magnets being used, but scaling measures of this type are only capable of influencing the actual magnetic penetration depth insignificantly. In addition, the NMR device becomes unusable for portable use due to larger and thus heavier permanent magnets.
Another magnet arrangement for generating the most homogeneous possible stationary magnetic field for the use of NMR studies is described in U.S. Pat. No. 6,489,872, which essentially comprises two permanent magnets implemented as hollow cylinders, which are situated coaxially but spaced apart from one another axially. To perform an NMR study, at least one HF coil and a sample to be studied are provided between the two permanent magnets, that is, each outside the spatial areas enclosed by the particular hollow-cylindrical magnets.
A further alternative arrangement for generating an NMR-compatible permanent magnetic field is described in US Published Application 2006/0055404, in which according to one exemplary embodiment, a plurality of individual permanent magnets are situated on the concave side of a parabolic surface, which can preferably also be deformed in a controlled way. Using this magnet arrangement, it is possible in the course of constructive spatial superposition of a plurality of individual permanent magnetic fields to provide a local large elevated magnetic field within a very small spatial area, within which the magnetic field conditions required for the NMR study result.
A further NMR magnet arrangement also based on the above principle of constructive magnetic field superposition is described in US Published Application 2006/0097725, in which a plurality of cylindrical permanent magnets are situated annularly and axially inclined to one another, so that the individual magnetic fields are constructively superimposed to form an extensively homogeneous magnetic field along a shared axis of symmetry. Using an arrangement of this type, which allows a sample study accessible on one side, liquid samples may particularly advantageously be analyzed in the course of an NMR study.